Pyongyang claims a further breakthrough toward a nuclear-tipped missile that can reach American cities

By Jonathan Cheng

South Korean and U.S. missile systems firing in a joint drill Wednesday—‘countering North Korea’s destabilizing and unlawful actions on July 4,’ the two militaries said. Photo: Handout/Getty Images

SEOUL—The U.S. warned North Korea that it is ready to fight if provoked, as Pyongyang claimed another weapons-development breakthrough following its launch of an intercontinental ballistic missile a day earlier.The regime, having demonstrated its capacity to reach the U.S. with a missile, on Wednesday touted another achievement of the test launch: It claimed that its missile warhead—the forward section, which carries the explosive—can withstand the extreme heat and pressure of re-entering the earth’s atmosphere.If true—the claim couldn’t be independently verified—that would clear another hurdle in developing a nuclear-tipped missile that can reach American cities.As tensions between Washington and Pyongyang rose, Gen. Vincent Brooks, the top American military commander in South Korea, said in a statement Wednesday that the U.S. and South Korea are prepared to go to war with the North if given the order.“Self restraint, which is a choice, is all that separates armistice and war,” Gen. Brooks said. “We are able to change our choice when so ordered.…It would be a grave mistake for anyone to believe anything to the contrary.”Earlier in the day, allied armies conducted a rare live-fire drill, launching tactical surface-to-surface missiles off the east coast of Korea—an action they said was aimed directly at “countering North Korea’s destabilizing and unlawful actions on July 4.”The drill and tough language appeared meant to reassure Seoul after North Korea’s successful ICBM test, a significant advance.Secretary of State Rex Tillerson described the development as an escalation of the threat to the U.S. It came despite years of sanctions and warnings aimed at preventing Kim Jong Un’s regime from reaching the milestone.Washington has considered military action against North Korea, but pulling the trigger presents serious risks. Seoul, a city of 10 million, sits just 35 miles from the North Korean border, where Pyongyang has assembled artillery that could inflict devastating damage on the densely populated South Korean capital.“A single volley could deliver more than 350 metric tons of explosives across the South Korean capital, roughly the same amount of ordnance dropped by 11 B-52 bombers,” said a report published last year by Austin, Texas-based geopolitical consultancy Stratfor.If attacked by the U.S., North Korea would also likely fire on U.S. ally Japan, which is within range of many of Pyongyang’s missiles. During one launch in March the North fired four missiles at once toward Japan, which some analysts interpreted as a warning that it could overwhelm any Japanese missile defense.Meredith Sumpter, director of Asia for Eurasia Group, wrote in a note to clients Tuesday that the odds of a U.S. military strike on North Korea remain low—about a 10% probability—adding it would probably be well-signaled by the U.S. and “clear to outside observers in advance of any military move.”A report published Wednesday in North Korea’s official state media said the warhead its missile carried Tuesday maintained a steady temperature and held its structure even “during the harshest atmospheric re-entry environment.”The Threat From North Korea’s Missiles

Pyongyang has accelerated its tests of missiles and nuclear bombs as it tries to develop a nuclear-armed missile that could hit the U.S. mainland

For a missile to cross the Pacific Ocean, it must exit and then re-enter the atmosphere. Re-entry puts incredible stress on the warhead.The U.S. had sought Beijing’s help in pressuring North Korea, but recently President Donald Trump indicated that route had been fruitless. “Trade between China and North Korea grew almost 40% in the first quarter. So much for China working with us - but we had to give it a try!” he said in a tweet on Wednesday.A Chinese customs official told a news conference in April that China’s bilateral trade with North Korea in the first quarter had increased by 37.4% to 8.4 billion yuan (about $1.2 billion). He didn’t specify if that was a year-on-year comparison.Chinese customs figures show that bilateral trade continued to expand in April and May on a year-on-year basis, but that doesn’t mean more revenue for North Korea: The second-quarter increase was driven by China’s exports. Its imports from its neighbor declined in April and May, compared to those months last year, due in large part to Beijing enforcing a ban on North Korean coal.The United Nations Security Council is scheduled to hold a meeting on North Korea later Wednesday, following a request from Nikki Haley, the U.S. ambassador to the U.N.South Korean President Moon Jae-in, who has called for more dialogue and closer economic ties with North Korea, on Wednesday called on global leaders to step up sanctions against North Korea, urging a peaceful resolution to the conflict.He was speaking during joint statements in Berlin with Chancellor Angela Merkel, who said North Korea “poses a big threat to global peace.”The U.S. has been making shows of force in recent months in response to perceived increases in tension on the Korean Peninsula. In April, it said it was sending the USS Carl Vinson carrier strike group to the western Pacific to underscore Washington’s commitment to the region. In that case, the announcement instead raised questions about U.S. credibility after it came to light that the aircraft carrier was thousands of miles away.And twice in May, the U.S. sent B-1B bombers on flyovers near the Korean Peninsula. Each came shortly after a North Korean missile test.—Jeremy Page in Beijing contributed to this article.

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